This invention relates to optical diffraction gratings and, more specifically, to diffraction gratings which have spacings that can varied by application of a mechanical force.
Optical diffraction gratings are used in many applications from spectroscopy to fiber optic telecommunication devices to human visual correction. Diffraction gratings are important to these technologies because of their ability to separate, or disperse, polychromatic light into its monochromatic components. These monochromatic components, where visible light is diffracted, may be visible as a rainbow-like image. Generally, a diffraction grating is a collection of reflecting or transmitting elements separated by a distance that is on the order of the wavelength of the light of interest, or some fraction of that wavelength. These elements may be formed by transparent slits in an opaque screen, or by reflecting grooves on the surface of a substrate. Incident light, or more generally, electromagnetic radiation, falling upon a grating will be diffracted by the grating in a predictable manner such that the electric field amplitude or phase, or both, is modified according to the well known grating equation.
Monochromatic light falling upon a grating surface is diffracted into discrete directions, or angles. Each grating slit or groove may be characterized as a small, slit-shaped source of diffracted light. Light diffracted by each groove combines to form a diffractive wavefront such that, for each given spacing between the grooves or slits, the diffracted light from each groove or slit is in phase with the light diffracted from any other facet so that the light combines in a constructive fashion at a specific angle, and in a destructive fashion for other angles.
Typically, diffraction gratings have a fixed period or grating spacing. This fixed grating spacing may be disadvantageous in certain applications because such a grating is only able to diffract frequencies of light incident upon the grating into specific angles. This constraint requires other components of an optical system incorporating such a grating to be controllable to obtain useful and desirable results.
Various attempts at providing variable spacing diffraction gratings have been previously attempted. For example, Interscience, Inc. of Troy, N.Y. manufactures one type of reconfigurable diffraction grating using micro electromechanical system (MEMS) technology. The grating structure consists of a structure having deflectable beams that are manufactured with a thick spacing. This structure is mounted above a common lower electrode. When the lower electrode is energized, selected ones of the deflectable beams are attracted towards the underlying electrodes. Changing the vertical position of the selected beam with respect to other stationary beams presents a different ruling spacing distribution to incoming radiation. By changing this distribution, the diffracted power among individual diffraction orders of the wavelengths is altered.
One of the disadvantages of the above-described prior art system is that it is only capable of delivering a number of grating spacings that are dependent upon the number of deflectable beams that can be deflected. Moreover, such a device requires vertical movement of the deflectable beams which may be limited depending on the construction of the device.
What is needed and has been heretofore unavailable is a diffraction grating having variable spacing capable of being configured to diffract a relatively wide range of incident light wavelengths into variable angles that can be easily and inexpensively manufactured. The present invention satisfies these and other needs.
The invention provides a variable spacing diffraction grating which allows the spacing between the beam arms, or grating elements, of the grating to be varied to allow the grating to be adjusted to accommodate incident light having varying wavelengths to diffract the incident light into varying angles. Moreover, the variable spacing diffraction grating of the present invention may be manufactured using micro-electronic manufacturing techniques to provide a variable spacing diffraction grating that is relatively inexpensive to manufacture, is trouble free and reliable in use, and that can attain its variable spacing using computer control of micro-actuators.
In one aspect, the present invention includes a series of substantially parallel interconnected beams spaced apart to form a grating having a selected diffraction angle. The grating may be formed on a substrate in such a manner that each beam is free to move in relation to the substrate so that when a force is applied to a beam at one end of the grating, the beams will move in a coordinated manner and the distance between the beams will change to provide a different diffraction angle.
In another aspect of the present invention, the beams of the diffraction grating are formed as vertical bars connected to one another by horizontal members in such a way that the grating resembles a square wave in shape. In this embodiment, one end of the grating may be fixed, with the other end operably connected to an actuator. When the actuator applies a force to the end of the grating, the vertical bars of the grating move together, or farther apart, depending on the direction of the applied force. Alternatively, an actuator may be operably connected to each end of the grating to effect a change in distance between the vertical bars of the grating.
In a further aspect of the present invention, the beams of the variable spacing diffraction grating may be formed as spiral arms that are arranged in parallel adjacency to form a spiral grating. The inner ends of the spiral arms are connected approximately at the center of the spiral. The outer ends of the spiral arms may be connected to an actuator or actuators configured to apply a force on the end or ends of the spiral arms to change the distance, and thus the grating spacing, between the spiral arms.
In yet another aspect of the present invention, the grating may be formed using a plurality of steps arranged in a series of staircases. In this embodiment, each step in a staircase is offset from an adjacent step such that the angle formed by the offset is the blaze angle of the grating. The bottom step of each staircase is connected to the top step of the next staircase in the series. One end of the series of staircases may be fixed, and the other attached to an actuator. Alternatively, both ends may be attached to actuators. When force is applied to an end of the grating, the staircases of steps move closer together or further apart, depending on the direction of force applied. When the staircases move relative to each other, the spacing of the grating increases or decreases accordingly, allowing for the diffraction angle of the grating to be adjusted to diffract light into varying angles and to accommodate wider ranges of incident light wavelengths than current diffraction gratings.
Still another aspect of the present invention provides a reconfigurable dense wavelength division multiplexer utilizing a variable spacing diffraction grating to separate incident light including a range of multiplexed light wavelengths into individual light beams having only one wavelength. The reconfigurable dense wavelength division multiplexer may be adjusted in the field to accommodate varying multiplexed light having varying ranges of wavelengths. The present invention is advantageous in that it allows a technician setting up a telecommunication network utilizing fiber optic elements to use a single device to accommodate a range of multiplexed light where multiple devices would have been required previously.
In still further aspects of the present invention, variable spacing diffraction gratings embodying the present invention are used to construct channel selectable wavelength add/drop modules which may selectively add or drop wavelengths from a multiplexed light beam falling on the grating. Additionally, variable spacing diffraction gratings of the present invention may be used to replace the standard diffraction grating used in any optical device where channel or wavelength selectability would be useful. Such an application would include, besides those already described, for example, a wavelength selectable optical cross-connect. Such a device would allow a cross-connect to accept any range of input light and diffract the light so that any wavelength of input light may be directed to any output port of the cross-connect.